1. Field of the Invention
The present invention pertains to serial printing devices which print characters and images in multiple printing areas of a print medium by scanning printheads across the print medium. In particular, this invention provides for improved output from shuttle-type printing devices in which multiple printheads are disposed at a fixed distance from each other and wherein each printhead scans and prints over a divided section of a print medium.
2. Description of the Related Art
Some conventional printing devices utilize full-line printheads, which are capable of simultaneously printing an entire line of data upon a print medium. Unfortunately, such printheads are quite expensive.
In contrast, serial printing devices operate by scanning a printhead across a print medium. The printhead forms images upon the print medium as it is scanned across. Such printheads are required to print only a small amount of data at any one time and are therefore generally less expensive than full-line printheads. Accordingly, the serial printing method is widely utilized in various printing devices.
In a serial printing device, a printhead is capable of printing data on a relatively limited area at any one time. This area is defined by printing elements such as ink jet nozzles on the printhead. Accordingly, in order to print over an entire page, a serial printhead is placed on a carriage which scans over the page, thereby printing an image as a combination of separately-printed areas. These areas are commonly referred to as swathes, or bands.
In a generic serial printer, a discontinuity (banding) appears between adjacent bands each time adjacent bands are printed. The banding occurs in a generic serial printer because a distance that a print medium travels after one band is printed does not coincide with a width of the printhead. Therefore, a subsequently-printed band is not located in a correct area of the medium. Furthermore, even if the traveling distance of a print medium is the same as the width of a printhead, banding often occurs in a ink jet printing device because ink at the edge of bands tends to smear.
In addition, in a case where adjacent bands have a different density, the density difference is pronounced at a boundary between the bands. Density differences occur because bands are printed at different times or because of differences in temperatures of a printhead during printing of adjacent bands.
In a serial printing method in which a collection of bands forms an image, it is relatively difficult to improve printing speed. In this regard, dual-directional printing has been proposed. In such printing, bands are printed by a printhead during scanning in each of two opposite directions.
Although this dual-directional printing method improves printing speed by a factor of two in comparison to a single-directional serial printing method, image quality degrades because of slight differences in printing position between one scanning direction and the other. This registration misalignment occurs due to reasons such as backlash of a scanning mechanism.
A technique to address the banding problem by utilizing dual-directional printing is described in U.S. Pat. No. 5,044,796. According to this technique, upon detecting a break or gap in image data to be printed, printing of the data is performed in both scanning directions across the break. If a break is not detected in image data to be printed, printing is performed in only one scanning direction.
During serial printing, a printhead occasionally heats to an abnormal temperature. On such occasions, printing is interrupted in order to protect the printhead. After the printhead cools down to a certain temperature or, alternatively, after a certain length of time passes, printing is continued from the location at which the interruption took place. In such instances, image density differences occur at the interrupt location due to the difference in printhead temperature before and after the interruption. In addition, printhead registration may not exactly match before and after such an interruption, thereby causing a discontinuity to appear in a printed image along the direction of an array of printing elements.
It should be noted that banding due to temperature increases during printing is not limited to a printhead which utilizes heat, such as a thermal or bubble jet printhead. It may also occur with printheads utilizing actuator wires for dot-matrix printing.
In other instances, an electric power supply having a limited power capacity is used in order to reduce the cost of a printer power supply. Such a power supply allows all printing elements to be used simultaneously to print on a single scanning path only when an image to be printed has a low density, as in the case of a text-based document image. On the contrary, such a power supply does not allow all printing elements to be used simultaneously on a single scanning path in a case where an image to be printed has a high density, such as a picture.
For a high-density image, a band which could be printed by all printing elements is sub-divided in order to reduce power consumption during one scanning path. For example, half of the printing elements are used during a first scan and a second scan, using the remaining elements, completes printing of the band. In such a case, a density difference or another type of discontinuity may appear between the sub-bands printed during the two scans.
U.S. Pat. 4,272,771 and corresponding Japanese Laid-Open Patent Application No. 50-81437 describe a method for improving the printing speed of a serial image printing device. According to these documents, the left and right halves of each printed line are printed simultaneously. To accomplish this simultaneous printing there are provided separate left and right printhead assemblies, both of which are supported by one common carriage mechanism. Accordingly, print speed is doubled over similar single-printhead systems. Furthermore, these documents claim that further increases in printing speed can be achieved by using more than two printhead assemblies or by printing in both directions of the scanning movements.
It should be noted that such multiple-printhead systems experience problems similar to those described above with respect to conventional single-printhead serial devices.
In particular, registration adjustment between printheads is quite important in a printing device having multiple printheads which print simultaneously on a same print medium. If the vertical registration (transverse registration) is not adjusted correctly, a vertical mismatch will occur in an image printed by the left and right printheads. This mismatch is very noticeable at a boundary between the two areas printed by the left and right printheads. If the horizontal registration (lateral registration) is not adjusted correctly, the two areas printed by the left and right printheads become separated or overlapped.
Moreover, areas adjacent to the boundary between the printing areas of the left and right printheads are printed at different times. Hence, as described above, the time difference creates recognizable density differences between the areas. Furthermore, if the left and right printhead are at different temperatures, a corresponding density difference appears at a boundary between the areas. In addition, recognizable density differences occur due to differences in output density characteristics of the left and right printheads.
In order to address the above problems, Japanese Laid-Open Patent Application No. 6-270488 discloses a technique to alter the area divisions corresponding to left and right printheads or left and right scanning motions by detecting a break in an image which runs in the direction of image columns (the direction of an array of printing elements).
The techniques in the description of U.S. Pat. No. 5,044,796 or Laid-Open Patent Application No. 6-270488 are effective when an image consists of simple characters and therefore breaks run in a direction of a printed character row or a column. However, the techniques are not effective when an image does not contain breaks. Examples of an image without breaks are those containing special characters, such as underlined characters, or a picture. These techniques also fail to address detection of breaks which are skewed with respect to column or row direction, and breaks which are not linear. Furthermore, these techniques do not disclose any method for detecting breaks in multiple-bit (multilevel) image data.
Moreover, as described above with respect to the serial printing method, an image suffers from density differences, banding, and registration misalignment between adjacent bands either along the scanning direction of printheads or in the direction of a printing element array.
Therefore, while density differences in an image based on temperature, time, or printhead output density characteristics can be corrected to some extent using known techniques, further improvement is desired. Also, while banding can be corrected to some extent by adjusting image density at the edge of bands, further improvement is desired. For registration misalignment, while it can be compensated to some extent by mechanical or electrical adjustment, further improvement is also desired.
One purpose of this invention is to provide a printing device, a printing method and a printer driver which produce a printed image having little image degradation, in a case that the image is composed of a combination of multiple printable areas.
Another purpose of this invention is to provide a printing device, a printing method and a printer driver which produce a printed image with little noticeable density differences, banding, and registration misalignment, in a case that the image is composed of a combination of multiple printable areas.
In order to achieve the above purposes, this invention is characterized by a printing system in which an image is printed within multiple printable areas of a recording medium, the multiple printable areas defining an overlapped printing area, and which includes detection of a break position in image data to be printed within the overlapped printing area using at least one of a plurality of break detection systems on the image data to be printed within the overlapped printing area, selective division of, at the detected break position and among the multiple printable areas, the image data to be printed within the overlapped printing area, and printing of the divided image data within the multiple printable areas.
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached Figures.